This invention is directed generally to ionography, and more specifically, to electroreceptors for ionographic imaging.
In ionography, latent images are formed by depositing ions in a prescribed pattern onto an electroreceptor surface. The ions may be applied by a linear array of ion emitting devices or ion heads, creating a latent electrostatic image. Alternatively, the electroreceptor surface may be charged to a uniform polarity, and portions discharged with an opposite polarity to form a latent image. Charged toner particles are then passed over these latent images, causing the toner particles to remain where a charge has previously been deposited. This developed image is sequentially transferred to a substrate such as paper, and permanently affixed thereto.
Ionography is, in some respects, similar to the more familiar form of imaging used in electrophotography. However, the two types of imaging are fundamentally different. In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The electrophotographic plate is insulating in the dark and conductive in light. The radiation therefore selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. Thus, charge is permitted to flow through the imaging member. The electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reuseable photoconductive insulating layers.
Ionographic imaging members differ in many respects from the above-described and other electrophotographic imaging members. The imaging member of an ionographic device is electrically insulating so that charge applied thereto does not disappear prior to development. Charge flow through the imaging member is undesirable since charge may become trapped, resulting in a failure of the device. Ionographic receivers possess negligible, if any, photosensitivity. The absence of photosensitivity provides considerable advantages in ionographic applications. For example, the electroreceptor enclosure does not have to be completely impermeable to light, and radiant fusing can be used without having to shield the receptor from stray radiation. Also, the level of charge decay (the loss of surface potential due to charge redistribution or opposite charge recombination) in these ionographic receivers is characteristically low, thus providing a constant voltage profile on the receiver surface over extended time periods.
An advantage of ionography over electrophotography is its elimination of the need for a photoreceptor. In its place, a non-light sensitive dielectric receptor of appropriate dielectric constant and thickness is used to retain the latent electrostatic image formed by controlled ion deposition onto its surface.
After the imaging and development steps in an ionographic imaging process, it is necessary to remove (erase) any remaining charge and/or toner to prepare the electroreceptor for the next imaging cycle. The erase function in dielectric receptors is typically performed by exposing the imaging surface to corona discharge to neutralize any residual charge on the imaging device. Thus, the erase function depends upon the contact with the top surface of the electroreceptor. For example, U.S. Pat. No. 4,137,537 to Takahasi et al discloses an electrostatic transfer process and apparatus wherein image forming areas on an insulating surface of the apparatus are erased by electric discharge from closely spaced pin electrodes.
This erase function of ionographic imaging members is subject to failure. Internal polarization and/or trapped charges can result in failure of the erase function due to dielectric relaxation effects. These problems have been encountered in ionographic printers, for example, manifesting themselves in the form of ghost image artifacts in prints. Further, corona discharge generates ozone and other undesirable effluents.
In electrophotographic systems, on the other hand, the erase function is achieved by photogeneration of copious amounts of charge carriers in the photoreceptor by erase light exposure of a wavelength from about 400 to 800 nanometers. Although residual potential cycle up can occur in electrophotographic imaging members, magnitudes and rates of residual formation are typically much less than the dielectric relaxation potentials observed in some ionographic receptors.